Conventional flush valves used with water closets, urinals and the like are typically not designed to deliver the same fixed flow volume of water during each flushing cycle independent of the line pressure to which they are subject. That is, typically with such flush valves, as the line pressure increases the amount of water discharged during a flushing cycle increases. This causes problems. First, with increase in pressure water is wasted, since any water discharged in addition to a predetermined required flushing volume is unnecessary. Second, when the line pressure decreases, the volume of water discharged during the flushing action decreases and may be insufficient to fully perform the flushing action and empty the water closet or urinal. In order to overcome this problem, certain flush valves may be purchased with a second, separate unit, which includes an adjustment feature. This requires the added expense inherent in the purchase of the second unit, and also the expense of the additional installation work which must be done by a plumber. Installation services may typically be a significant factor in the overall cost. Perhaps even more important, however, is the fact that such units must be set by the installer/plumber at the time of installation so that they produce sufficient volume of water for each flushing cycle. This leaves to the plumber the decision of just how much water is sufficient to complete a flushing cycle. In addition, the adjustment is made for the line pressure which is the line at the moment that the adjustment is made. Subsequent fluctuations in line pressure will cause variations in the discharge volume, since these fluctuations impose pressures other than the specific pressure for which the adjustment was made.